The use of membranes for separation processes has been known for some tens of years. Such separation processes were based on the selective permeation of different moieties through thin membranes. In many of the systems the separation was due to the selectivity of permeation due to size differences of the various species in the mixture. The first processes were used for the separation of colloidal species and for the separation of other suspended small size particles from liquids. Amongst the first types of membranes there may be mentioned membranes with rather large and non-homogeneous pore diameter (&gt;10 .ANG.), which were satisfactory for the separation of mixtures with very large size differences of the species to be separated. A later development relates to reverse osmosis membranes used for the sweetening of brackish water. In parallel, ion exchange membranes were used for separation processes based on electrodialysis.
Amongst the important developments there may be mentioned the provision of hollow fiber membranes resulting in a very large overall membrane surface area in a small module, providing also the possibility of easy sealing of the edges of such modules.
A further development relates to the invention of the assymmetric membrane which comprises a coarse pore, thick support to which there is applied a very thin, dense and highly selective membrane, which thin membrane is used for the actual separation process, while the coarse support provides the required mechanical strength. Conventional processes for the separation of a certain gas or gases from gaseous mixtures are based on differences in chemical or physical properties, of the various constituents.
Cherical processes involve cyclic processes where certain chemicals are used for the removal of constituents. An example is the removal of hydrogen sulfide from natural gas. This may be effected by the iron sponge method where a reactor comprising a bed of finely divided iron is used in a single run, leaving iron sulfide wastes. Another process is the cyclic amine method where an organic ammonium salt is obtained during natural gas purification, which has to he thermally decomposed so as to recycle the amine. Amongst the most common physical separation methods are those based on fractional distillations. The most common is the separation of nitrogen and oxygen from air. On a large scale (about 1000 ton/day) the multistage cryogenic process is economical from an energy point of view. On a smaller scale other processes are more economical. There are also used adsorption processes based on the different adsorbability of various components of the gas mixture. Such process is advantageously effected as a multistage cyclic process in column configuration. After use, the adsorbent has to be regenerated for recycling, which can be effected at low pressure or by heating. Various separation processes are based on the use of specific zeolites or of carbon molecular sieves.